The invention relates generally to a method and device to convert chemical reaction energy directly into electric power, and more specifically to a method and device to generate vibrationally highly excited reaction products and to convert the product energy directly into useful forms, such as electricity, hot electrons in a metal surface, long lived carriers in a semiconductor, radiation or coherent radiation.
The fuel cell is a clean and efficient electrochemical energy conversion method that typically converts chemical reaction energy of reactant gases directly and efficiently into electricity. The power per mass or per volume exhibited by a fuel cell system, however, is typically at least an order of magnitude lower than that of mechanical engines. Further, the volume needed for a fuel cell using liquid, storable fuels to generate the power of a given battery is typically significantly larger than that of the battery. This means that the present form of fuel cells cannot replace the battery regardless of the fuel cell efficiency, because there is no room for fuel.
In addition, the type of fuel cell with the highest known power per mass, the solid oxide fuel cell, operates at temperatures of 600 to 800 Celsius. Operation at this temperature presents materials problems as a result. In principle, this fuel cell would exhibit the power density required to operate in the volume of a battery it would displace. Thermal issues, however, have dominated and prevented realization of this goal.
The alternative to a high performance fuel cell is a rotating mechanical device. However, mechanical engines generating electricity must typically use coil and magnet devices to convert mechanical energy into electrical energy and are therefore relatively heavy, with power densities less than 2 watts per gram.
Therefore, there is a need to have a method and system for converting chemical energy directly and efficiently into electricity and to have methods that do not require high temperatures and materials and that do not require relatively heavy mechanical devices.
A chemical reaction typically creates highly vibrationally excited specie, which reaction may be stimulated by catalysts, injection of autocatalysts, or other means. A substantial fraction of the excitation energy may be transferred to an energetic electron in a metal when the excited specie comes in contact with the metal. See Huang, Yuhui; Charles T. Rettner, Daniel J. Auerbach, Alec M. Wodtke, Science, Vol. 290, 6 October 2000, pp 111-113, xe2x80x9cVibrational Promotion of Electron Transfer.xe2x80x9d
According to results of experiments, vibrationally excited anions may absorb an electron and re-emit an electron into the lattice, carrying with it most of the excitation energy. By analogy, cations may emit an electron and reabsorb it, emitting a hole into the lattice, with the hole carrying the energy. The electron or hole is the hot carrier.
Recent experimental observations and theoretical developments in surface science confirmed that even relatively weakly electronegative gas molecules vibrating with an energy almost sufficient to break their chemical bonds (vibration quantum number in excess of order 15) can deposit a majority of the vibration energy into an electron of the metal surface during a single, brief contact (of order 0.1 picoseconds) with that surface. Research and observations associated with understanding this observation support the theory of prompt, multi-quantum energy transfer to a single electron from a vibrationally excited chemical specie.
Typically, more than half of the vibrational mode energy will be transferred directly into an electron of the metal surface with an energy greater than approximately 5 vibrational quanta. The result is that an electron in a metal surface may carry away a substantial, useful fraction of the vibrationally excited molecule energy as a hot electron, also referred to as a hot carrier.
In the metal, the hot electron may travel into a semiconductor. The hot electron becomes converted into an excitation or potential difference in the semiconductor where it may be converted into other useful forms such as an electrical potential driving a current in an external circuit, an inverted population of semiconductor excitations, or hot carriers transported to other locations for use.
The hot electron may be converted into a potential in a semiconductor. For example, U.S. Pat. No. 6,222,116 collects such hot electrons directly without mechanical means, and converts them directly into electricity when the vibrationally excited chemical product specie is formed on or within a few molecule dimensions of the reaction surface of its device. The device described in this patent generates useful electricity by causing the reactions rates to be sufficiently high to energize the semiconductor converter into maintaining a useful forward bias.
A removal of spent reaction products through desorption from a hot-electron collection surface may enhance this high reaction rate. De-energized molecules desorbing may leave behind a clean site for more reactions. Allowing these de-energized molecules to migrate away from the conducting surface may initiate further reaction with more oxidizers and fuels.
Accordingly, there is a need to have a method to produce vibrationally highly excited specie directly from chemical reactions and where the conversion of the electrical energy of the excited specie may take place at a location different from in the thermal sense and separated from the creation of the excited specie. Further, there is a further need to have reactions take place in a volume, not just a surface, to increase the rate of reaction compared to that on a surface.
In one aspect, a method is provided to use chemical reactants to produce vibrationally excited gas molecules in a reaction volume or region, and to extract a substantial fraction of energy, for example, as hot electrons in a conducting surface, and to convert the energy, such as hot electrons, into useful forms of energy.
The method includes using chemical reactions to generate vibrationally excited molecules such as high quantum number vibrationally excited gas molecules. The vibration energy in the vibrationally excited molecules is converted into hot electrons when the excited molecules contact a conductor. A geometry such as a gas reaction region with dimensions of order several molecular collision mean free paths and bounded in part by a conducting surface is provided so that the excited molecules may travel, diffuse or wander into a conductor before loosing a useful fraction of the vibrational energy. Optionally, the generating and the converting process may be thermally separated, at least in part. The short lived hot electrons are converted into longer lived entities such as carriers and potentials in a semiconductor, where the energy is converted into a useful form.
In another aspect, an apparatus for generating energy provided includes a reaction region where the reactants, for example, fuel and oxidizer, undergo chemical reactions that produce highly vibrationally excited molecules. Exhausts produced in the reaction are allowed to leave the system. A reaction region may be close enough to the collection surface so that the excited products do not loose a substantial fraction of their energy before reaching the collection surface. A collection surface may include a surface such as a conductor near or on which excited products transfer energy to a hot electron or carrier (electron or hole). A conversion region may be in contact with the collection surface, where the hot electron or carrier is converted into a useful form, such as a potential in a semiconductor sustained by separated carriers. This conversion region may be at least partly isolated at least thermally from the reaction region.
Yet in another aspect, the method may include transferring reaction product excitations such as multiple-quantum change of energy of a dipole active state of a vibrational state, which are a type of excitation of the energized chemical products.
Further features and advantages of the invention as well as the structure and operation of various embodiments of the invention are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.